Asthma and other respiratory diseases have long been treated by the inhalation of appropriate medicament. For many years the two most widely used and convenient choices of treatment have been the inhalation of medicament from a drug solution or suspension in a metered dose pressurised inhaler (MDI), or inhalation of powdered drug generally admixed with an excipient, from a dry powder inhaler (DPI).
Inhalation activatable dispensers for use with aerosol containers which contain medicament and are pressurised with liquid propellants and are equipped with a motoring valve through which a plurality of metered doses may be dispensed are known, their general purpose being to afford proper co-ordination of the dispensing of a dose of medicament with the inhalation of the patient thereby allowing the maximum proportion of the dose of medicament to be drawn into the patient's bronchial passages. Examples of such dispensers are described in British Patent Specification Nos. 269,554, 1,335,378; 1,392,192, 2,061,116 and 2,240,930 U.S. Pat. Nos. 3,456,644, 3,456,645, 3,456,646, 3,565,070, 3,598,294, 3,814,297, 3,605,738, 3,732,864, 3,636,949, 3,789,843 and 3,187,748 and German Patent No. 3,040,641.
European Patent No. 147028 discloses an inhalation activatable dispenser for use with an aerosol container in which a latch mechanism releasing vane is pivotally mounted in an air passage between an aerosol outlet valve and a mouthpiece, which latch mechanism cannot be released if force to activate the dispenser is not applied before a patient inhales.
The dispenser generally comprises a housing having a mouthpiece and an air passage therethrough terminating at the mouthpiece, the housing being adapted to receive an aerosol container and having a support block with a socket adapted to receive the stem of the valve of the aerosol container and a through orifice communicating between the socket and the air passage, and latch means having parts movable between an engaged position in which movement of the container and the support block toward each other upon the application of a force to bias the container and the support block toward each other is prevented and a release position in which movement of the container and the support block toward each other in response to said force is permitted causing the stem to move to its inner discharge position, the latch means comprising a vane mounted on the housing in the air passageway between the orifice and the mouthpiece for movement toward the mouthpiece under the influence of inhalation through the mouthpiece to release the latch means in which the vane moves toward the mouthpiece from a blocking to a non-blocking position with respect to the passageway in response to inhaling at the mouthpiece and releases the latch means only during the application of said force to bias the container and support block toward each other.
This inhalation device has been received favourably by patients and doctors since it not only overcomes the hand-lung co-ordination problem but it does so at a very low triggering flow-rate (approximately 30 liters/minute) essentially silently, and with a very compact design barely larger than a standard inhaler.
Many inhalation activatable dispensers utilise a spring to bias the container relative to the valve stem and ultimately move the container relative to the valve stem to fire the device upon inhalation. Generally the spring acts upon the base of the container and is compressed prior to patient inhalation by a priming force generated by operation of a priming lever which may be a separate lever or connected to a pivoting mouthpiece cover. In some arrangements, after activation of the device and removal of the priming force the valve and associated triggering mechanism is reset under the influence of the internal valve spring by movement of the container relative to the valve stem. Thus, in order to ensure efficient operation of the dispenser it is necessary to carefully select the parameters of the biasing spring used to prime the device.
The spring configuration for such a dispenser must exhibit certain specific force characteristics. These characteristics are best described with reference to the three main stages of operation of the device as follows:
The unprimed or at rest stage in which no priming force is applied to the spring and the spring is applying its lowest load to the container. In this position, ideally, the force applied by the spring should be insignificant in the sense that it has no influence on the position of the aerosol container and valve thus allowing the device to reset under the influence of the internal valve spring following the previous actuation. Thus the force applied by the biasing spring must be less than the minimum valve return force.
The priming stage in which the spring is compressed. The force applied by the spring must be greater than that required to fire the aerosol valve but should be kept to a minimum as it governs the magnitude of the inspiratory flow rate required to actuate the dispenser.
The firing stage in which the container is moved relative to the valve sufficiently by the spring to actuate the valve. For correct operation, it is essential that as the spring expands from its primed position it retains sufficient force to overcome the inherent resistance to movement of the valve. Failure at this stage would result in no delivery of medication,
The theoretical spring force profile i.e. a plot of force against spring length, is a straight line. In practice, the rate is not linear but tends to increase with compression (the softest parts of the spring Compress first). In order to achieve the parameters required for successful dispenser operation, the spring must be selected such that requirements of the unprimed and fired positions are met and in practice this can mean springs generate undesirably high forces in the primed position. Thus, spring selection and manufacturing tolerances are critical.